MEMS technology is being actively developed by many organizations for implementing components and systems for application in Radio Frequency (RF) systems. The reasons for this interest relate to the distinct advantages the MEMS technology provides over competing technologies, including lower cost and higher performance.
One device that is extremely useful in RF systems is the tunable or variable capacitor component, in which the value of the capacitance of the device can be adjusted using some external means. Most prior embodiments of variable capacitors have been in the form of discrete components, in which the capacitance of the device is adjusted by turning a screw that is coupled to variably overlapping capacitor electrodes. These devices are typically manually adjusted once on the factory floor to achieve a desired “tuned” capacitance value and then left in that state for the life of the product.
The prior discrete variable capacitor technology has several drawbacks as discussed herein. First, the prior variable capacitors are prone to drifting from the desired capacitance value to some other undesired capacitance value over time due to temperature or other environmental effects. This is because the prior variable capacitors are essentially mechanical elements composed of a variety of materials, including a high conductance metal for the electrodes and electrical interconnections, a dielectric positioned between the electrodes, and a mechanical adjustment mechanism by which the overlap of the capacitor electrodes can be adjusted over some range. In addition, each of the materials used for the capacitors has a different coefficient of thermal expansion. Consequently, the materials will expand or contract at different rates, depending on the ambient temperature, thereby resulting in a relative movement of the components in the device made from the different materials.
Second, prior variable capacitors have no capability for active tuning of the device capacitance before or during operational use. This means that once the component value of capacitance has been adjusted on the factory floor, there is no means or mechanism by which the desired capacitance value can be actively maintained or adjusted.
Third, the process of tuning prior variable capacitor devices to a desired capacitance value is a manual operation usually involving a technician, and thus is time intensive and costly. Typically, the technician must make a proper electrical connection to the device or circuit in which the device is located with some sort of test and measurement apparatus, and while monitoring some parameter, adjust the variable capacitor to a desired value by turning a screwdriver mechanism.
Fourth, the process for adjusting capacitors to a desired value is prone to error in a production line environment. Frequently, the capacitor may be turned to the wrong capacitance value. This error is typically detected in later stage testing and usually can be corrected by subsequent tuning adjustment; but, this increases cost significantly. Alternatively, the device out of specification can be scraped, but this is expensive as well. Ultimately, the decision of whether to savage or scrape out-of-spec devices will depend on the economics of a given situation.
Fifth, the prior technology of variable capacitors has a very limited tuning range. This is primarily due to the fact that the mechanical movement is typically limited to a linear motion, and as a result, the tuning ratio (i.e., the ratio of minimum to maximum capacitance values over the entire dynamic range of the device) is generally limited to 10 to 1 or less. The disadvantage of this is that a much larger number of different variable capacitor devices with a slightly different tuning range must be made in order to have capacitors adequately cover the entire range of possible continuous capacitance values. Since almost any value of variable capacitance is desired in practical applications, the result is significantly higher design and manufacturing costs, as well as high inventory costs, as would not be the case with a smaller number of variable capacitors with a larger dynamic range.
Sixth, the current versions of variable capacitors are large in size, and therefore, consume a larger amount of space on the mounting substrates, such as Printed Circuit Boards (PCBs), ceramic substrates, etc. As electronics technology has advanced, almost every other component or system, when viewed at the board or substrate level, has radically decreased in footprint, with the exception of variable capacitors. Consequently, there is the need for variable capacitor devices which are smaller in size.
Seventh, the maximum operating frequency and the quality factors (Q) of the prior variable capacitors are somewhat limited. This is a particular problem as RF systems using these components have steadily moved into higher operational frequency ranges and the performance demands for higher signal to noise ratio and lower power consumption have increased significantly.
Finally, current variable capacitor manufacturing technology has matured to the point where the cost of making the components has been reduced substantially since the introduction of these devices decades ago. However, it is expected that very little improvement in cost reduction will be made in manufacturing these components in the future. Furthermore, despite the cost reduction achievements that have been made, these devices are still relatively expensive when compared to other components. Moreover, as pointed out above, there is a substantial additional cost associated with installing and tuning these devices compared to other components.
Additionally, there is a competing market of discrete surface-mount capacitors, which are available in fixed incremental steps of capacitance values (e.g., 0.1 pF, 0.2 pF, 0.5 pF, etc.), but the absolute value of capacitance of these devices varies over a fairly large range from the manufacturer. That is, if one desires a fixed capacitor of the value 1 pF, the manufacturer can only provide capacitors within a percentage range of the “desired” capacitance value. There is a need for making discrete capacitors having a much improved tolerance of capacitor values.
Although MEMS-based variable capacitors have been previously developed, all of these devices have been realized on traditional semiconductor materials, i.e., primarily silicon wafers. While this approach works for the demonstration of a device, it presents several major disadvantages for the commercialization of discrete microwave or high-frequency RF devices. First, the resistive ohmic losses of the silicon substrate are very high at high operational radio frequencies, i.e., at frequencies above 1 GHz. Second, the cost of silicon substrates and the processes used to fabricate variable capacitor devices on these substrates are too high, compared to existing technologies. Third, the packaging costs of silicon, or other semiconductor material bases, are very high, particularly for devices that must operate at high frequencies and under extreme environmental conditions.
While the losses of the silicon substrate can be reduced appreciably by selectively removing the silicon from under the active devices and the associated signal paths using an isotropic etchant, such as Xenon Diflouride (XeF2), this is an expensive process and one that is not readily compatible with the commercial fabrication of active MEMS devices. Consequently, the resulting manufacturing yield will be low and the cost will increase appreciably. Other semiconductor substrates having lower resistive losses can be employed for the fabrication of MEMS devices, such as Gallium Arsenide (GaAs), resulting in high performance devices. However, the cost of these materials and the costs to fabricate devices on these materials are typically two orders of magnitude higher than silicon wafers and processes. Consequently, the resulting device cost will be substantially higher than the existing macroscale variable capacitor devices. Furthermore, any semiconductor-based variable capacitor device will require a separate packaging technology that will need to be specifically developed to meet the demanding requirements of a commercial product. A conventional die package that will meet the required specifications and simultaneously have a low cost is not readily possible with today's technology.
There is enormous opportunity for MEMS technology in the application of variable capacitors, and if cost and performance goals can be met, the potential market sizes for these devices will be enormous. Also, there is also an enormous opportunity for a micromachining process on discrete capacitors whereby the capacitors can be made inexpensively but having extremely tight tolerances on the capacitance values. However, to exploit these opportunities, there is a need for new low cost and low resistive loss material onto which MEMS devices can be successfully fabricated with high yields. These is also a need to inexpensively be able to accurately and precisely tune these capacitors to a desired capacitance value and have them remain at that value permanently and without electrical stimulation. Furthermore, there is a need for the capability to suitably and inexpensively package these variable capacitor devices.